[Bash-autocompletion] Add support for older bash version.

Summary:
OS X seems to use older bash version which doesn't suport
_init_completion and compopt, so add support for this.

Reviewers: ruiu, v.g.vassilev, teemperor

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34924

llvm-svn: 306962